The present invention relates to devices for regulating the flight characteristics of airfoils and, more particularly, to use of a spoiler mechanism operative to selectively disrupt airflow across the airfoil upper surface to regulate the lift characteristics of the airfoil in flight.
Lift characteristics of an airfoil depend on a variety of features, including the size and shape of the wing, and other factors. Lift characteristics are important factors in determining performance characteristics of an aircraft, such as range, payload, and engine requirements. By designing an airfoil to have high lift characteristics, engine requirements may be reduced. Range and pay load characteristics may also be extended as a result of lift characteristics. Consequently, there are good reasons to design an airfoil to have high lift characteristics.
Some aircraft, such as unmanned aerial vehicles (UAVs), are designed to optimize lift, range, and payload characteristics, while avoiding the need for powerful, heavy engines and control mechanisms. This allows a UAV to fly substantial distances, and loiter for extended periods, without running out of fuel.
There are, however, circumstances where high lift characteristics may introduce problems in the operations of such vehicles. For example, one of the critical phases of UAV flight is landing, or other recovery, e.g., by flying into a net. High lift characteristics of the UAV may dictate that the UAV descent angle, or glide angle, is relatively shallow such that the UAV takes considerable time and distance before it is able to descend from its operating altitude to a landing or capture location. Indeed, such characteristics may preclude landing the UAV at locations where such glide angles are inconsistent with the terrain or other operating conditions, i.e., where a steeper descent angle is necessary, or otherwise more advantageous.
Accordingly, while the advantages of a high lift vehicle may enhance certain operating characteristics of various types of aircraft, there are circumstances where those lift characteristics need to be altered in order to achieve more optimum operation of the aircraft. Preferably that alteration may be done in a temporary manner during only a portion of the flight profile.
Lift characteristics can be modified by creating an interference, or airflow protuberance, which cause disturbance or perturbation of airflow about the wing upper surface. The desired perturbation causes the flow to separate from the top of the airfoil and a controlled aerodynamic stall condition to occur over the portion of the airfoil behind the lift spoiler. This greatly reduces the lift produced by that section of the airfoil.
Various proposals have been made to introduce such protuberances. One approach is to provide a spoiler mechanism that translates to a position above the wing, into the flow, where it impedes the airflow across the wing, causing the boundary layer to separate from the top of the airfoil which results in reduced lift. While such a technique is useful for its intended function, it requires that the spoiler translate to a position in the airflow where it is exposed to, and must withstand the force of airflow across the wing. The structure and power requirements to withstand such forces typically results in the need to use stronger motors and more sturdy mechanisms that are relatively heavy and expensive. In the case of some light aircraft, such as UAVs, those requirements can be prohibitive.
Another approach has been to provide a porous surface along a portion of the airfoil upper surface. When an adjacent inner surface is translated a proper distance, the porous upper surface is opened to an inner cavity to allow airflow through. That airflow disturbs the airflow across the airfoil upper surface and decreases lift. Again, such mechanisms are suitable for their purpose, but suffer from limitations. For example, the mechanism for translating the inner surface may be difficult to precisely maintain and operate. Moreover, airflow patterns across the wing may encounter interference, even when the perforations are closed, due to the irregularities in the outer surface and spacing between the outer surface and the translatable inner surface. Further, where low observability characteristics are desired, it is preferable to avoid surface irregularities that may detract from those characteristics.
Accordingly, it is desirable to provide an airfoil that, in normal operation, exhibits high lift characteristics, but may also include a mechanism that is operative to selectively introduce drag that reduces the lift characteristics, and allows for different modes of operation, without significantly adding to the weight requirements or significantly degrading the low observability characteristics.
It is preferable that such an airfoil be constructed in a manner such that, in normal flight, the outer surface is uninterrupted by any surface irregularities, and that deployment of the spoiler mechanism does not require that any structure be extended directly into the airflow, which could impose significant weight/cost penalties and degrade low observable characteristics.
These and other objects and advantages are addressed and achieved in accordance with the present invention, as described and illustrated herein.